1. Field of the Invention
The present invention relates to a photographic lens for a camera, and more particularly, to a photographic lens which has improved aberrations and is compact so as to be suitable for electronic still cameras, vehicle mounted cameras, PC cameras, mobile devices such as cameraphones, or mobile information devices, for example, PDAs which use a solid state image sensing device such as a charge-coupled device (CCD) or a complementary metal-oxide-semiconductor (CMOS).
2. Description of the Related Art
Recently, there is a strong demand for both an increase in the number of pixels of the imaging module of a cameraphone and a decrease in the size of such cameraphones. With the development of a solid state image sensing device, the injection angle of a main light ray corresponds to a range of 20-25°, and accordingly, a plurality of thin and compact lenses are under development. However, the pixel pitch of a photographic device has decreased to be less than 2 μm, and thus, an increase in the basic resolution of a photographic lens is strongly required. As such, it is important to provide a photographic lens that is thin and compact and simultaneously has a high resolution.
In this regard, Japanese Patent Publication Nos. 2003-322792, 2004-163849, 2004-212467, 2004-240074, 2004-326097, and 2005-284153 disclose photographic lenses formed of three lens units. However, these photographic lenses fail to provide the desired resolution because the correction of chromatism is insufficient for a pixel pitch of 2 μm or less.
Also, Japanese Patent Publication Nos. 2002-228922, 2002-365529, 2002-365530, 2002-365531, 2004-341501, and 2004-341512 disclose photographic lenses formed of four lens units. These photographic lenses appropriately correct chromatism, and thus, increase resolution for pixel pitch of 2 μm or less using the four lens units. However, since the length of the overall lens system as compared to the diagonal length of the solid state image sensing device is increased, these photographic lenses are not satisfactory for thin and compact lenses.
Reducing the back focal length (Bf) is one of the solution to make a thin and compact lens. However, when the Bf is excessively reduced, glass such as an infrared filter cannot be inserted in the rear portion of the lens or the amount of space allowed during assembly is reduced. Furthermore, when the injection angle is increased, a shading phenomenon such as a distortion of contrast occurs when making a thin and compact lens.